You are basically trying to tamp down the linear physics a bit to make it less sensitive, it can be hard to find the right sweet spot if it is a lower mass vehicle, especially if it's mesh or high prim where most of the prims are not affecting the physics weight.

You will want to edit a few things directly in the vehicle physics inside the Root Settings. Take a backup copy of your vehicle first in case you need to restore your previous settings.

Line numbers refer to ACSv6 but are similarly placed in other versions:

Things to try:

If using mesh/high prim, make sure your vehicle has enough mass, so it doesn't bounce off every tiny bump. Make your root prim as large as possible (without blocking sit targets). Any "solid"/Convex Hull prim gives your vehicle more weight, this includes the root. If you have a tiny root and only solid wheels, the vehicle could just be way too light. You can even weigh it down with additional invisible cubes set to solid, these simple shapes will add minimal Land Impact.

Make sure your vehicle isn't skating/riding on the bottom of your animation bounding box (animation bounding box can interfere with motorcycle bank/lean as well). If raising up your sit position abnormally high changes how the vehicle behaves, you will need to deal with the animation bounding box.

Increase gravity

Lower the Z value in the vector for VEHICLE_LINEAR_FRICTION_TIMESCALE on line 122 of the v6 Root Settings.

You can also try increasing the VEHICLE_VERTICAL_ATTRACTION_TIMESCALE on line 125 a bit, as it will make it respond less vigorously to ground bumps

Bring down VEHICLE_LINEAR_DEFLECTION_EFFICIENCY a bit from 1.0 on line 115

If your tighten up the physics to make the vehicle less reactive in the Root Settings as described above, you may need to increase steering power and/or wheelie power to compensate.